Aircraft wind power plant

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A prototype of a kite from SkySails

An airborne wind turbine , even high-altitude wind power plant or dragon power plant called, is a wind power plant , fly's part in the air and are anchored only by one or more guys on the ground. The electrical energy can be obtained either with mechanical movement transmission with generators on the ground or with generators in the air (so far the highest output of test systems here was 600 kW).

One advantage is that the holding structure is subjected to tension rather than pressure and bending, so that the cost of materials can be lower than with conventional wind turbines with the same output power. In addition, airborne wind power plants can operate at higher altitudes than fixed wind generators, where the wind or high-altitude wind blows considerably stronger and more steadily than near the ground. The take-off and landing of aircraft wind power plants are problematic, especially in extreme weather conditions (storm, calm).

potential

Conventional wind turbines are limited to the use of wind close to the ground due to the hub height of the wind turbine and the rotor diameter. The conventional wind power plants that existed in 2010 can use winds up to around 200 meters above the ground.

The average wind speed increases with the distance from the ground where the wind is slowed down by friction, especially in rough ground surfaces such as hills, forests or tall buildings. The influence of this braking effect on the ground surface decreases with the distance from the ground. The wind speed increases up to a height of 10 km. At this altitude, the so-called jet streams with peak wind speeds of several hundred km / h occur in the moderate latitudes .

The average wind speed on the ground is around 5 m / s, whereas in the jet streams it is 40 m / s. The usable wind power increases with the third power of the wind speed. This means that if the wind speed is doubled, the power contained in the wind increases eightfold, if it triples it increases to 27 times. The average energy density in the jet streams at 40 m / s wind speed is 512 times as high as on the ground, neglecting the significantly lower air density at a height of 10 km. This potential can be estimated with the help of a global atlas published in 2008 on the energy density of high-altitude winds at different heights between 80 and 12,000 meters. However, it must be taken into account that the higher wind speeds require a higher mechanical strength of the systems and thus a modified design.

Airborne wind power plants could be operated at varying heights below a maximum altitude, that is, in weak winds in certain layers of air, the power plant could be allowed to evade. Wind energy can also be used at different heights at the same location, so that the usable wind power per unit of floor area is multiplied compared to conventional wind turbines. In this way, significantly higher amounts of energy could be drawn on a small area of ​​land, the land consumption and the impact on the landscape would be lower.

The permanent wind at higher altitudes also means higher utilization of the wind turbines. The capacity factor of terrestrial wind turbines, for example, averages around 30-40% depending on the location, while projections for high-altitude wind turbines assume a capacity utilization of up to 80%. This effect would improve the steadiness of wind energy and thus alleviate a significant problem in the use of wind energy. The compulsion to keep alternative power sources available, mostly using fossil fuels such as coal or natural gas , would be eased. The electricity production costs could also possibly decrease due to the higher utilization of the power plants. As there are no commercial aircraft wind power plants yet, no real data are available.

Due to the higher average wind speeds and the lower dependence on the nature of the ground and the strength of the winds close to the ground, high-altitude wind power plants can also be operated at locations unsuitable for conventional wind power plants, for example in inland areas with little wind. Locations for wind power plants could therefore be based on the electricity consumption in the region and less on the wind speed on the ground. This could reduce the effort required to expand the grid to convert the energy supply to wind energy, which is necessary in Germany for the transport of wind power from the windy generating areas in the north to the consumption centers in the middle and south of Germany. However, it must be taken into account that flying wind turbines would have a significantly greater impact on aviation , which limits the possible locations.

Estimates assume that the producer prices of less than 1 euro cent per kWh of electricity up to 2 cents per kWh of electricity could be realistic. If this assessment turns out to be correct, aero-wind power plants would not only be by far the cheapest regenerative energy source, but also cheaper than fossil- fuel power plants, even without taking into account their subsequent costs such as CO 2 pollution .

Construction principles

In contrast to conventional wind turbines, an airborne wind power plant is not attached to a tower or mast to achieve the high winds, but is merely held in place by ropes. The airborne wind power plant floats - either because it is lighter than air or because it creates aerodynamic lift . The following design options arise:

  • Balloon-like wind power plants filled with light gases would be lighter than air, so they would float without dynamic lift. An example of such a concept is a cylindrical balloon floating transversely in the wind, rotatably mounted on its longitudinal axis, which, thanks to the arched lamellae attached to the long side, rotates like an anemometer or a Savonius rotor and drives generators on the sides.
  • Aircraft wind power plants, which are heavier than air, convert part of the wind energy into dynamic lift that keeps them in the air. You need a wing or a kite sail and control surfaces ( tail unit ). These systems place significantly higher demands on the control, since a control error could lead to a crash.
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Furthermore, a distinction must be made between power plants that fly or hover stationary over a place and those that increase the energy yield by flying as quickly as possible across the wind (cross-wind power), for example in circular movements or in the form of an eighth flight maneuver , which are also used in kite surfing , increase the area swept by the wing, the relative wind speed on the wing and the wind power that can be used by the power plant. The underlying principle can be easily explained by comparing it with conventional wind turbines. With these, the tips of the wings generate the majority of the power because they move the fastest and thus cover a large area. The wind is slowed down on the entire swept area and not only on the section of the circle on which the wing is currently located. Designers of such high-speed flying wind power plants aim to reduce the wind power plant to these effective, then flying parts and to make the heavy and expensive remaining load-bearing parts such as the blade centers, the hub and the mast unnecessary.

Constructive differences to conventional wind power plants

Construction type and materials

The balloon-based design in the example of the TWIND system, which uses the vertical force and regulates the air resistance with the help of the parachute

The flying part of an aircraft wind power plant must be built lightly. In order to make this possible, in addition to the choice of textile and flexible building materials, constructions are also advantageous that only load the material with tension and pressure, but generate as little shear forces or moments as possible, since the latter require construction methods that are heavier.

Moments can be almost completely avoided by guy ropes, as is used for example with kites. However, bracing increases air resistance. Particularly if the power output is to be increased through fast flight, however, a low air resistance with high lift, i.e. a high glide ratio, is desirable. Therefore, there is a structural challenge in making the wing stable, light and yet aerodynamic. Inflatable structures with internal chambers or guy ropes can be used for this.

However, even with commercially available surf kites, considerable powers of 30 or 40 kW have been calculated.

Steering and control

The autonomous control of airborne wind power plants and the fact that, in contrast to conventional wind power plants, they fly freely in space, represents one of the central challenges in the development of airborne wind power plants. Previous developments have primarily failed because of this problem. Various sensors for measuring as many parameters as possible (wind speed and direction, position, relative and absolute speed, direction of movement, rope tension, vibrations, etc.) must be forwarded to an autopilot , which then carries out the correct steering maneuvers using control software. The software must be designed in such a way that it enables the safest possible flight while promoting the greatest possible energy production. Sudden and unforeseen changes in wind speed and direction pose a particular problem. There are also challenges in the take-off and landing phase, which requires a completely different flight movement than normal operation.

In the past, the lack of options in the field of sensor technology and computer capacities was one of the greatest obstacles in the construction of airborne wind power plants. In recent years, however, many advances have been made in this area

The actual flight control is carried out either, as in an airplane, by various (elevator, rudder, aileron) rudders attached to the airborne wind power plant, or, in accordance with the control of kites, by shortening the steering cords and ropes and thus by changing the position of the wing. With the latter variant, all control cables can either be led from the wing to the ground station, with increased air resistance and delayed response and less precise steering instructions to be expected with the corresponding cable length. An alternative is to bring the steering cables together on a steering module below the wing. The further connection to the ground station would then be made via a single rope. The steering module would then have to have an energy source to execute the steering movements. This would have to take place via accumulators , a power cable built into the rope or by small wind turbines on the wing that generate the working current.

Power generation

In principle, electricity can be generated in the air or at the ground station.

Electricity generation on the ground

When generating electricity on the ground, the generator is located in the ground station. The energy is transmitted mechanically, mostly via ropes, from the wing to the ground station.

The most favored variant is the so-called yo-yo configuration. The holding rope at the ground station drives a generator via a rope drum while the rope is being unwound. As soon as the end position is reached, the rope is hauled in again with motor power. The wing is positioned in such a way that it has the lowest possible air resistance and thus only little time and energy is required to retrieve the rope. Then the cycle starts all over again. This can result in a positive energy balance, i.e. H. more electrical energy is fed into the power grid than is consumed.

Other alternatives provide that the kinetic energy is transmitted to the ground by a rapidly rotating tether, which serves as a shaft.

Variants called Laddermill use ropes with several adjustable surfaces that either alternately or continuously drive generators on the ground via winches or rollers.

The advantages of generating electricity on the ground are the potentially lower weight and the potentially lower complexity and cost of the wing. The disadvantages are to be seen in the energy supply required for the wing as well as the lack of the possibility of autonomous take-off and landing according to the helicopter principle.

Electricity generation in the air

To generate electricity in the air, heavy generators and, if necessary, gears must be carried in addition to the rotor and tether. In heavier-than-air constructions, the wind flows through the rotor plane at an angle, so that part of the wind load causes the lift. The current can be conducted to the ground station via a ladder built into the rope. The generators are used as motors during take-off and landing .

A concept pursued by the company Skywindpower is similar to a conventional wind turbine flying stationary over a point. The connection of four counter-rotating rotors by a frame like a quadrocopter allows control over the inclination and the torque balance .

The company X Development LLC relies on the above-mentioned cross-wind principle with attached aircraft. The wing, which moves across the wind faster than the wind, can be understood as an aerodynamic gear. The propulsion is correspondingly smaller than the lift and is used by small rotors , such as airplane propellers, arranged perpendicular to the wing to generate electricity. Their high speed allows small, lightweight generators without any further mechanical transmission.

Challenges in development and operation

Use of airspace and risk of collision with aircraft

In the airspace above 100 meters there is competition and a risk of collision with aircraft , up to 1000 meters above all with private aviation. In order to ensure their safety, no-fly zones would have to be set up above the location of aircraft wind power plants , as is already the case above nuclear power plants and some other built-up areas.

Lightning strike and icing

Ropes, cables and electrical systems must be designed for lightning protection .

At high altitudes, there is a greater risk of icing. Even rotor blades in conventional systems are heated if necessary.

Crashes

Because of the complex control of airborne wind power plants, which are heavier than air, crashes are to be expected, at least in the test phase. Even with mature systems, crashes cannot be ruled out with absolute certainty. Aircraft wind power plants should therefore only be considered at locations where there is no risk to people.

History, projects and development

While the variants of the flying wind turbines are becoming increasingly technically mature, the use of the jet stream according to the above and today's principles does not yet seem tangible. Today's systems operate with rope lengths of a few hundred meters and flight heights of 300 to 500 meters.

Before 1900

In the past, kites were occasionally used to lift loads. The kite was developed in Asia. People were also lifted with kites - for amusement, but also for military observation. The principle of the kite came to Europe via Marco Polo . Leonardo da Vinci proposed a kite to cross a river, pull vehicles and divert the energy of lightning. Even before the invention of the motor vehicle , kites were used to pull carriages, for example by Benjamin Franklin , who also drove boats with kites.

1900 to 2000

The kite pioneer Samuel F. Cody crossed the English Channel in a boat pulled by a kite in 1903 and in the same year set the record for the highest kite flight with 14,000 feet (approx. 4,200 meters). With the inventions of powered flight and the use of fossil fuels , interest in the use of high-altitude winds waned until the oil crises of the 1970s, which led to renewed interest and various research projects. The engineer ML Loyd, for example, has examined the generation of energy by kites in detail. However, because of the drop in oil prices in the 1980s , these projects were largely abandoned together with other research projects in the field of alternative energies, such as experimental wind generators. From the 1990s on, research and development focused on conventional wind turbines.

It was not until the turn of the millennium that there was renewed interest in air wind power plants for generating electricity . Developments in the field of sensors, materials, computer-controlled autopilots, etc. make it possible to build and operate aircraft wind power plants. Since then, university research groups and non-university start-ups have been working on the development of aero-wind power plants, partly with the support of external donors.

In addition to many patents and publications in this area, from 2009 annual international conferences on aircraft wind power plants will also be held; for example at the University of Freiburg, Institute for Microsystems Technology in the technical faculty, at the beginning of October 2017.

2000 to 2010

In 2001 the company SkySails was founded.

Makani was founded in 2006 and has been listed as the Google X Project since 2013 . In 2007, for example, the Makani company received approx. 30 million USD from a subsidiary of the Google Group for the development of an aircraft wind power plant.The Makani kite energy system uses autonomous connected wings that fly in a circular path and generate electricity via wind turbines that are on the main wing are mounted.

Until 2009, the completely autonomous flight operation of an aircraft wind power plant over several days with autonomous take-offs and landings had not yet succeeded.

2010 to 2020

The NASA since 2010 explored the possibility of flying wind turbines.

European developments, such as the Brandenburg company EnerKite, the Brandenburg company NTS, the University of Delft in the Netherlands, and the Italian company Kitegen deal with stunt kites which drive generators on the ground. On March 23, 2012, EnerKite put a mobile aircraft wind turbine into operation with a nominal output of 30 kW, which is used for technology development and demonstration.

Another concept, also similar to a stunt kite, was developed by the Californian company Joby Energy . The company Makani, also from California, continues these developments and presented a 30 kW system with a propelled 8 meter wide wing; Here the energy is converted into the air by wind generators on the wing and conducted to the ground with high voltage through a rope of constant length. This was followed in 2014 by a prototype with a span of 26 meters and an output of 600 kW, which was to be developed with the help of Shell to supply offshore platforms and which had its first test deployments in the North Sea in 2019.

The company Altaeros Energies has a prototype called Buoyant Airborne Turbine developed (BAT), in which the wind turbine is in a helium-filled tube. According to Spiegel , the wind turbine has a diameter of 3.7 meters; the inflatable tube is 15 meters long and just as wide.

2020 to 2030

At the beginning of 2020, Alphabet , the Google parent company, announced that it would end the Makani project, as a successful and sustainable business model apparently could not be developed from it, the uncertainties and risks were too high.

literature

Web links

Videos

Remarks

  1. ↑ a b Werner Pluta: Alphabet gives up flying wind turbine. February 19, 2020, accessed February 19, 2020 .
  2. ↑ a b Wind energy use with fast flying kites: a challenge for optimization and control; D. Diehl, B. Houska in automation technology 2009, page 525 (531) (complete PDF downloadable via link, German).
  3. ↑ Modeling, Simulation, and Testing of Surf Kites for Power Generation, P. Williams, B. Lansdorp, R. Ruiterkamp, ​​W. Ockels in AIAA Modeling and Simulation Technologies Conference and Exhibits 2008, Honolulu, Hawaii, page 2.
  4. ↑ Cristina L. Archer, Ken Caldeira: Global Assessment of High-Altitude Wind Power. Energies 2009, doi: 10.3390 / en20200307 (free full text).
  5. ↑ a b c d Erik Vance, Wind power: High hopes , in: Nature 460, (2009), 564-566, doi : 10.1038 / 460564a .
  6. ↑ a b c M. Canale, L. Fagiano, M. Milanese Power Kites for Wind Energy Generation . IEEE Control Systems Magazine, page 25 (27, 28).
  7. ↑ - ( Memento from March 29, 2018 in the Internet Archive ) Information base of the Münster University of Applied Sciences
  8. ^ Pressure Induced Stability: From Pneumatic Structures to Tensairity . Archived from the original on April 26, 2013. Retrieved March 28, 2018.
  9. ^ J. Breuer, W. Ockels, RH Luchsinger: An inflatable wing using the principle of Tensairity . In: 48th AIAA Structures, Structural Dynamics and Materials Conference, Waikiki, Hawaii, USA, 2007 . 2007 ( tudelft.nl [accessed June 10, 2020]).
  10. ↑ SkySails worked with this variant.
  11. ↑ https://www.researchgate.net/publication/40822587_Robust_Control_of_Laddermill_Wind_Energy_System AR Podgaets, WJ Ockels: Robust Control of Laddermill Wind Energy System , accessed on March 28, 2018
  12. ↑ Sky Windpower. Retrieved June 10, 2020 .
  13. ↑ Technology | Makani. Retrieved June 10, 2020 .
  14. ↑ Dragons are keen on wind energy . In: vdi-nachrichten.com . March 30, 2012 ( vdi-nachrichten.com [accessed April 12, 2017]).
  15. ↑ a b c d Past, Present and Future of Kites and Energy Generation; J. Breukels, Wubbo Ockels.
  16. ↑ ML Loyd, Crosswind Kite Power in Journal of Energy, 4 (3), page 106 ff.
  17. ↑ See a list of patents in the English language Wikipedia .
  18. ^ Conference of the year 2015
  19. ↑ a b Alphabet pulls the plug from the wind energy kite developer Makani. In: https://www.it-times.de/ . February 20, 2020, accessed July 4, 2020 .
  20. ↑ USD 15 million in 2007 and later another financing “of this magnitude”: High Hopes; Erik Vance in Nature 2009, Volume 460, Page 564 (566)  ( page no longer available , search in web archives ).@1@ 2Template: Dead Link / www.kuleuven.be
  21. ↑ NASA is investigating the possibility of wind farms at lofty heights derstandard.at, December 21, 2010
  22. ↑ An Answer to Green Energy Could Be in the Air nasa.gov, December 10, 2010, accessed December 22, 2010
  23. ↑ Alexander Bormann: EnerKíte - Aircraft wind power plants. Retrieved April 12, 2017 .
  24. ↑ NTS X-Wind - The economic energy transition. Retrieved April 22, 2017 .
  25. ↑ Alok Jha: Giant kites to tap power of the high wind . In: The Guardian . August 2, 2008, ISSN  0261-3077 ( theguardian.com [accessed April 12, 2017]).
  26. ↑ Dragons are keen on wind energy . In: vdi-nachrichten.com . March 30, 2012 ( vdi-nachrichten.com [accessed April 12, 2017]).
  27. ↑ EnerKíte. Retrieved April 22, 2017 .
  28. ^ Joby Energy. (No longer available online.) Archived from the original on April 20, 2017 ; Retrieved April 22, 2017 .
  29. ↑ Energy Kites. Retrieved April 22, 2017 .
  30. ↑ Balloon technology: wind turbine supplies electricity from a height of 600 meters. Spiegel Online , February 6, 2015, accessed February 16, 2015 .
  31. ↑ BAT: The Buoyant Airborne Turbine. Altaeros Energies, accessed February 16, 2015 .
  32. ↑ Golem, February 19, 2020: Alphabet gives up flying wind turbine